Method and apparatus for mass spectrograph evacuation with respect to an atmosphere-interchange relationship



W. M. HICKAM R ss SPECTROGRAPH EVACUA s RE-INTERGHANGE RELATIO Sept. 28, 1965 METHOD AND APPARATUS RESPECT TO AN A Filed Aug. 27, 1962 NOv INVENTOR William M. Hickum BY 2 W.

ATTORNEYE WITNESZnM Zl-r/ z -w Se t. 28, 1965 w. M. HICKAM 3,209,144

METHOD AND APPARATUS FOR MASS SPECTROG'RAPH EVACUATION WITH RESPECT TO AN ATMOSPHERE-INTERGHANGE RELATIONSHIP Filed Aug. 27, 1962 5 Sheets-Sheet 2 Sept. 28, 1965 w. M. HICKAM 3,209,144

METHOD AND APPARATUS FOR MASS SPECTROGRAPH EVACUATION WITH RESPECT TO AN ATMOSPHERE-INTERCHANGE RELATIONSHIP Filed Aug. 27, 1962 5 Sheets-Sheet 3 VELOCITY SELECTOR EVACUATING BEAM GENERATOR TO MECHAN ICAL PUMP ASSEMBLY 3,209,144 H EVACUATION WITH RELATIONSHIP 5 Sheets-Sheet 4 W. M. HICKAM S F MASS SPECTROGRAP PHERE-INTERCHANGE ARATU AN Sept. 28, 1965 METHOD AND APP RESPECT T Filed Aug. 27, 1962 mokom mwo 20.

p 28, 1965 w. M. HICKAM 3,209,144

METHOD AND APPARATUS FOR MASS SPECTROGRAPH EVACUATION WITH RESPECT TO AN ATMOSPHERE-INTERCHANGE RELATIONSHIP Filed Aug. 27, 1962 5 Sheets-Sheet 5 Fig.2D.

United States Patent 3,209,144 METHOD AND APPARATUS FOR MASS SPECTRO- GRAPI-I EVACUATION WITH RESPECT TO AN ATMOSPHERE-INTERCHANGE RELATIONSHIP William M. Hickam, Pittsburgh, Pa., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Aug. 27, 1962, Ser. No. 219,664

8 Claims. (Cl. 250-41.9)

This invention relates to electromagnetic apparatus and has particular relationship to mass-spectrograph or massspectrometer apparatus. This invention is applicable both to spectrographs and to spectrometers and is embodied interchangeably in both. Reference made herein to spectrographs or spectrometers alone shall be understood to apply to both.

It is broadly an object of this invention to provide a highly sensitive and highly precise mass spectrograph or spectrometer particularly suitable for the analysis for the presence and/or the quantity of very minute traces (for example, one part inten'million) of certain important substances in specimens particularly of high-purity materials such as those used in the nuclear and solid-state industries.

This invention is in its broader aspects applicable to mass spectrograph apparatus of all types both those in which the source is in the form of an injected gas or vapor and those in which the source of the ions under analysis is of the spark type, but in its more specific aspects, this invention is particularly applicable to, and has particular advantages in, mass spectrographs in which the ion source is a spark. Typically, this invention applies to a mass spectrograph as disclosed in application Serial No. 150,447, filed November 6, 1961, to William M. Hickam and George G. Sweeney for Electromagnetic Apparatus. This Hickam-Sweeney application is included herein by reference.

In mass spectrograph apparatus in accordance with the teachings of the prior art, a beam of the ions of the specimen under test is produced in a chamber which is relatively highly evacuated by the combination of a mechanical pump and an ion-gettering pump or diffusion pump with the usual nitrogen trap. The ion beam in this chamber is deflected in accordance with the respective masses of the ions and a spectrum of the deflected beam is produced or the ion currents for the respective masses are measured by electronic means.

It has been found, particularly in situations in which the source of the ions is a spark produced between specimen wires, that the apparatus does not have adequate sensitivity and reliability to produce positive indications of minute quantities of materials which are sought by the mass spectrographic analysis. It is a specific object of this invention to produce mass spectrographic apparatus with which positive indications of materials which are present in only minute quantities in the specimen shall be produced.

This invention arises from the discovery that the insufiiciency of the sensitivity and the reliability of the prior art apparatus is caused by the excessive diffusion of the ion beam and by the presence in the beam of quantities of gaseous or vaporous materials such as oxygen, nitrogen, water-vapor, carbon dioxide, argon, hydrogen and neon. In additions, vapors from the mechanical pump or from any diffusion pump which may be used in evacuating the ion chamber are also present. While very small quantities of these materials are present in the chamber, these quantities are of the same, or even greater than, the quantities of the materials sought by the analysis.

In accordance with this invention in its specific aspects, highly sensitive and highly reliable mass-spectrograph ap- 3,209,144 Patented Sept. 28, 1965 paratus is achieved by a low-temperature pump which precipitates the above-mentioned gases and vapors and thus frees the ion chamber of their presence. The ion beam is sharp and does not include the ions of the abovedescribed vapors and gases. The spectrograph then contains clear and precise indications of the materials present in minute quantities which are sought by the analysis.

In accordance with the preferred practice of this invention the mass spectrograph apparatus includes a liquid helium pump. Liquid helium has a temperature of about 4 K. and in the practice of this invention a surface cooled by liquid helium to about 4 K. is in atmosphere-interchange relationship with the region in which the ion beam is produced.

The important aspect of this invention is the suppression of the vapors of gases of the atmosphere from the ionized beam. Such gases include hydrogen, oxygen, nitrogen, carbon dioxide, argon, neon and water vapor. The boiling points of these gases are as follows: hydrogen 20 K., oxygen 93 K., nitrogen 83 K., carbon dioxide l K., argon 88 K., neon 28 K., and water 373 K. The vapor pressures of these components which measures the extent to which these components are present in the ion beam is reduced to very low magnitudes by subjecting the region of the ion beam to the surface cooled by liquid helium. The pressure of hydrogen is reduced to 10* torr and the pressure of nitrogen and oxygen to 10 torr. The partial vapor pressures of the other components is correspondingly reduced. A torr is defined as the pressure at one millimeter of mercury. The above-described low-temperature pump thus has the advantage of concentrating the beam and removing confusion ions in the beam so that materials present in very minute quantities in the specimen can be detected.

In addition, the liquid helium pump has the advantage of high speed evacuation as compared to a diffusion pump or the like, the low-temperature pump above-described has the advantage that it permits the use of a much smaller pumping-atmosphere-intercharge area. The maximum pumping speed achievable with a diffusion pump is between 3 and 4 liters per second per centimeter squared of area of the jets. The number of molecules which would at the speed of molecules at room temperature pass through a centimeter squared area permits a maximum pumping speed of approximately 12 liters per second assuming that no molecules return and the liquid helium pump can approach this speed if the reflection coefiicient of the helium cooled surface is small (return of molecules small).

In the practice of this invention in its specific aspects, a pump requiring about 250 cubic centimeters of liquid helium to cool a surface was used. The cooled surface available had an area of approximately 16 centimeters squared and a pumping speed of l6 12 or 192 liters per second could be achieved, but because the cross sectional area of the atmosphere-interchange tube between the ionbeam chamber and the cooled surface was small a flow of only about 20 liters per second was actually achieved. The use of a surface cooled by liquid helium has important advantages in the practice of this invention since it eliminates substantially all gases which could militate against the precision and reliability of the spectrographs produced from the ion beam. In situations in which reasonable reliance can be placed on the absence of such gases as hydrogen and neon from the region of the beam the surface in atmosphere-interchange relationship with the region of the beam may in accordance with the broader aspects of the invention be cooled by liquid hydrogen or neon.

The novel features considered characteristic of this invention are described generally above. For a more thorough understanding of the invention both as to its organ- (3 ization and as to its method of operation together with additional objects and advantages thereof, reference is made to the following description taken in conjunction with the accompanying drawings, in which:

FIGURE 1 is a view in perspective showing a preferred embodiment of this invention; and

FIGS. 2A, 2B, 2C and 2D together constitute a view in section of the apparatus shown in FIG. 1.

The apparatus shown in the drawings is a mass spectograph or mass spectrometer including a Beam Generator which produces an ion beam in a chamber. Within the chamber a high vacuum substantially free of vapors of hydrogen, oxygen, nitrogen, carbon dioxide, argon, neon and water is maintained by an Evacuating Unit. This mass spectrograph also includes a Velocity Selector, a Monitor Electrode, an Ion Deflector and a Photographic Plate Assembly. The Beam Generator, Velocity Selector, Monitor Electrode, Ion Deflector and Photographic Plate Assembly are substantially the same as the corresponding components of the above-identified Hickam-Sweeney application. Ions from the spark S between the electrodes E1 and E2 of a material or specimen under test are accelerated in the Beam Generator to form a beam. The ions in the beam may be either positive or negative depending on the material under analysis. The beam is transmitted through the Velocity Selector which operates electrostatically to spread the beam in accordance with the velocities of the ions. The Velocity Selector may be set to select a predetermined range of ion velocities for analysis. The ions selected are concentrated in a sub-beam which is transmitted through the Monitor Electrode which in the practice of this invention in its more specific aspects is a slit or screen of progressively varying area. The beam transmitted by the Monitor Electrode is deflected magnetically by the Ion Deflector onto the photographic plate of the Photographic Plate Assembly (in a mass spectrograph). Ions of different masses impinge on diflferent parts of the plate in dependence upon their masses.

The ion beam is produced in a highly evacuated chamher or housing of a material such as stainless steel including a plurality of wall sections each joined to the end of another. These include a generally cylindrical section 21 in which the Beam Generator is enclosed, and a section 23 terminating in a part 25 extending at an obtuse angle to the section 21. The section 23 houses the Velocity Selector and the Monitor Electrode. To the section 23 a section 27 is connected; this extends largely at an obtuse angle to the part 25 and houses the Ion Deflector and Photographic Plate Assembly. The cylindrical section 21 is closed at its end by a plate or header 29 which carries the spark electrode assembly. The section 23 is sealed vacuum-tight to the section 21, the section 27 to the section 23 and the plate 29 to the end of the section 21. To assure a vacuum-tight enclosure Teflon seals are provided at these joints. The section 27 is closed at the end by a cross-wall section 31 which has a slot 33 for inserting the photographic plate 35 and its holder. A plate 37 is mounted on the wall 31 and carries a plug 39 for sealing the slot 33. At the end remote from the plate 29 the section 21 has a generally square portion 183 having port holes 41, 42, 44. Port holes 42 and 44 serve an exhaust opening for evacuation of the chamber and are connected to the Evacuating Unit. The section 23 has an exhaust opening 43 at the end near section 21. The section 27 has an opening 45 near the section 31. This opening 45 may serve for connecting an electrometer (not shown) or for additional exhaust. The cylinder 21 also includes a window 46 through wich the spark 7 may be observed.

The Beam Generator includes the spark electrodes E1 and E2 which are adjustably mounted in tubes 51 extending at an angle to the plate 29 and welded to this plate. Each tube 51 has secured thereto a generally elongated cup-shaped extension 53 having a shouldered opening 55 in its cap 57. Each electrode E1 and E2 is held in a pinvice 61 screwed onto a stem 63 which extends into an insulating glass bead 65. Another rod or stem 67 extends from the bead 65 through the end 69 of the tube 51 into the tube or sleeve 53. A sylphon or bellows 71 is mounted on the end 69. This sylphon 71 has a movable head 73 from which inner and outer stems 75 and 77 extend. The rod 67 is secured in the inner stem 75. A threaded rod 79 is screwed into the stem 77 and locked by a nut 81. The rod 79 passes through a swivel joint 83 in the cap 57 and sleeve 85 engaging the joint and is rotatable by a thumb-nut 87 engaging the sleeve 85 into which it is screwed.

The swivel joint 83 includes a two-part block 91 within which a ball 93 is enclosed. The ball 93 is movable within the block 91. The block 91 is held slidably within the cap 57 by a cover 95. The cap 57 has projections 97 and 99 at right angles each enclosing a spring 101 within a sleeve 103. Screws 105 and 107 opposite to the projections screw into the cap 57 and serve to set the position of the block 91.

The end plate or header 29 includes tubes 111 (FIG. 1) carrying seals 113 through which the hollow conductors 115 which carry the high frequency power to the spark S are sealed. The spark potential may have a frequency of the order of 800 kilocycles and a potential of 100 kilovolts.

The Beam Generator includes a conducting ring 121. This ring 121 is supported by L-shaped brackets 123 and 125 supported internally by the header 29 on insulating supports 127. Direct-current beam power of between 5,000 and 20,000 volts (typically about 12,000) is supplied through a conductor 131 which insulatingly passes through one of the tubular conductors 115 and is connected to one of the pin-vices 61. This pin-vice is also connected to the ring 121. The other terminal of the beam-power supply is grounded as is the housing. The ring 121 accelerates the ions in the spark S. The beam potential is positive if the ions are positive and negative if the ions are negative.

The Beam Generator also includes an accelerating electrode assembly 135 (FIG. 2B) which extends from the outwardly projecting stem 137 of an inner header 139. The accelerating assembly 135 includes a sleeve 141 secured to the stem 157. A ring 143 is secured to the end of the sleeve 141 and a cup-shaped cap 145 having a small central opening 147 is secured to this ring 143. The ring 143 also carries a plate 148 having an opening 149 coaxial with the opening 147. Another plate 151 having an opening 153 coaxial with the openings 147 and 149 is carried by the header 139 at the base of the stern 137. The openings 147, 149, 153 cooperate to collimate the ion beam. A protective screen 157 is suspended from the inner header 139.

The inner header 139 includes an outwardly extending stern 161 from which the Velocity Selector is suspended. The Velocity Selector includes an electrostatic field plate assembly .163 and a slit 165 for selecting the velocity range of the ion beam to be transmitted. The electrostatic field plate assembly 163 is curved and extends into the portion 25 of the section 23 which is bent at an angle.

The field plate assembly includes curved supporting plates 171 suspended from opposite fiat areas on the stem 161. Curved electrostatic plates 173 are secured to brackets 175 mounted on insulating (glass) blocks 177 between the plates 171. The surfaces of the plates 173 are at right angles to the surfaces of the plates 171. About 600 to 800 volts are impressed between the plates 173. The connection is effected through port hole 41 in square section 183.

The Velocity Selector also includes a disc 191 having a hole in its center. This disc 191 is mounted in a shouldered block 193 at the end of the supporting plates -171. The hole in disc 191 determines the ion velocity region of the beam.

The Monitor Electrode ordinarily used in the practice of this invention is the one disclosed in FIGS. 7, 8, 9

7. Mass spectrograph apparatus 'for producing a mass spectrograph of a specimen comprising a chamber, means connected to said chamber for mounting said specimen in said chamber, means connected to said specimen for producing a directed beam of ions of said specimen, and means for deflecting said beam in dependence upon the respective masses of the ions in said beam, the said apparatus being characterized by said evacuating means including mechanical pumping means and residual-gas pumping means, selectively operable first valve means between said mechanical pumping means and said chamber, selectively operable second valve means between said residual-gas pumping means and said chamber and selectively operable third valve means between said residualgas pumping means and said mechanical pumping means.

8. Mass spectrograph apparatus for producing a mass spectrognaph of a specimen comp-rising a chamber, means connected to said chamber for mounting said specimen in said chamber, means connected to said specimen for producing a directed beam of ions of said specimen, and means for deflecting said beam in dependence upon the respective masses of the ions in said beam, the said apparatus being characterized by evacuating means connected in atmosphere-interchange relationship with said chamber and including mechanical pumping means and residual-gas pumping means, and a connection between said residual-gas pumping means and said mechanical pumping means for removing the gases accumulated in said residual-gas pumping means during operation.

References Cited by the Examiner UNITED STATES PATENTS 2,547,173 4/51 Rittner 20163 2,622,204 12/52 Shaw 250-419 2,699,505 1/55 Usher 25041.9 2,712,223 7/55 Hunt 621 2,727,995 12/55 Loevinger 25041.9 2,967,239 1/61 Zemany 250-419 3,006,157 10/61 Haettinger 62259 RALPH G. NILSON, Primary Examiner. 

1. MASS SPECTROGRAPH APPARATUS FOR PRODUCING A MASS SPECTRUM OF A SPECIMEN COMPRISING A CHAMBER, MEANS CONNECTED TO SAID CHAMBER FOR EVACUATING SAID CHAMBER, MEANS CONNECTED TO SAID CHAMBER FOR MOUNTING SAID SPECIMEN IN SAID CHAMBER, MEANS CONNECTED TO SAID SPECIMEN FOR PRODUCING A DIRECTED BEAM OF IONS OF SAID SPECIMENT, AND MEANS FOR DEFLECTING SAID BEAM IN DEPENDENCE UPON THE RESPECTIVE MASSES OF THE IONS IN SAID BEAMS, THE SAID APPARATUS BEING CHARACTERIZED BY EVACUATING MEANS, CONNECTED IN ATMOSPHERE-INTERCHANGE RELATIONSHIP WITH SAID CHAMBER, AND INCLUDING A LIQUID-HELIUM PUMP FOR REDUCING THE PARTIAL VAPOR PRESSURE OF GASES AND VAPORS IN THE REGION OF SAID BEAM TO A VERY LOW MAGNITUDE. 